A communication system for communication by serial communication using photocouplers is known. As an example, Patent Literature 1 discloses a multi-split air conditioner including indoor units and an outdoor unit that communicate with one another by serial communication through a transmission path including a signal line and a reference line. In the multi-split air conditioner disclosed in Patent Literature 1, a transmission phototransistor included in a transmission photocoupler and a reception light-emitting diode included in a reception photocoupler are both provided on the transmission path.
Thus, in the multi-split air conditioner disclosed in Patent Literature 1, the reception photocoupler transitions from an OFF state (non-conductive state) to an ON state (conductive state) in response to a state transition of the transmission photocoupler from an OFF state to an ON state, whereas the reception photocoupler transitions from the ON state to the OFF state in response to a state transition of the transmission photocoupler from the ON state to the OFF state. Here, a delay time from a state transition of a transmission light-emitting diode included in the transmission photocoupler to a state transition of a reception phototransistor included in the reception photocoupler is longer than or equal to the sum of a switching time period for the transmission photocoupler and a switching time period for the reception photocoupler.